Implantable stimulation devices deliver electrical stimuli to nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder subluxation, etc. The description that follows will generally focus on the use of the invention within a Spinal Cord Stimulation (SCS) system, such as that disclosed in U.S. Pat. No. 6,516,227. However, the present invention may find applicability with any implantable medical device or in any implantable medical device system.
An SCS system typically includes an Implantable Pulse Generator (IPG), such as that described in U.S. Provisional Patent Application Ser. No. 61/874,194, entitled “Construction for an Implantable Medical Device Employing an Internal Support Structure,” filed Sep. 5, 2013, which is incorporated herein by reference. The IPG 10 of the '194 Application is shown in FIG. 1 in plan and cross sectional views, and includes a biocompatible device case 30 that holds the circuitry 27 and battery 34 necessary for the IPG to function. The IPG 10 is coupled to electrodes 16 via one or more electrode leads 14 that form an electrode array 12. The electrodes 16 are carried on a flexible body 18, which also houses the individual signal wires 20 coupled to each electrode 16. The signal wires 20 are also coupled to proximal contacts 22, which are insertable into lead connectors 24 fixed in a header 28 on the IPG 10, which the header can comprise an epoxy for example. Once inserted, the proximal contacts 22 connect to header contacts 26 in the lead connectors 24, which header contacts 26 are in turn coupled by feedthrough pins 48 (FIG. 2) to circuitry within the case 30. In the illustrated embodiment, there are sixteen electrodes 16 split between two leads 14, although the number of leads and electrodes is application specific and can vary. In an SCS application, electrode leads 14 are typically implanted on the right and left side of the dura within a patient's spinal cord. The proximal contacts 22 are then tunneled through the patient's tissue to a distant location where the IPG case 30 is implanted, at which point they are coupled to the lead connectors 24.
FIG. 2 shows perspective views of the bottom and top sides of the IPG 10 with the case 30 removed so that internal components can be seen, including the battery 34, a communication coil (antenna) 40, and a printed circuit board (PCB) 42. As explained in the '194 Application, these components are affixed to and integrated using a rigid (e.g., plastic) support structure 38. Battery 34 in this example is a permanent, non-wirelessly-rechargeable battery. (Battery 34 could also be rechargeable, in which case either communication coil 40 or another recharging coil would be used to wirelessly receive a charging field that is rectified to charge the battery 34). The communication coil 40 enables bi-directional communication between the IPG 10 and a device external to the patient (FIG. 3) via magnetic induction. The ends of communication coil 40 are soldered to coil pins 44 molded into the support structure 38 to facilitate the communication coil 40's eventual connection to the PCB 42. PCB 42 integrates the various circuitry 27 needed for operation of the IPG 10. Communication coil 40 is proximate to the bottom side of the IPG 10 in plane 40p, while the PCB 42 is proximate to the top side in plane 42p, as shown in the cross section of FIG. 1.
FIG. 3 shows an external controller 100 with a coil 108 for communicating with the IPG 10's communication coil 40 via a magnetic induction link 90. External controller 100 is preferably hand-holdable and portable, and includes a user interface (a display, buttons, etc.) to allow a user to adjust the therapeutic current that the IPG 10 is providing (e.g., to increase or decrease the stimulation being provided, to change which electrodes are providing the stimulation, etc.), and to review status information reported by the IPG 10.
In traditional SCS systems, data is bi-directionally transmitted along link 90 using a Frequency Shift Keying (FSK) protocol, in which a serial string of bits is wirelessly transmitted at different frequencies around a center frequency (e.g., fc=125 kHz). For example, if a ‘0’ bit is to be transmitted to the IPG 10, control circuitry 102 in the external controller 100 (e.g., a microcontroller) provides that bit digitally to modulator/transmitter circuitry 104 in the external controller 100. The modulator/transmitter 104 tunes coil 108 to resonate at 121 kHz for example for a bit duration (e.g., 250 microseconds). This frequency is transmitted via link 90 to the communication coil 40 in the IPG 10, whose demodulator/receiver circuitry 49 decodes it per its frequency as a digital ‘0’, and reports it to the IPG's control circuitry 50 (e.g., a microcontroller) for interpretation. A ‘1’ bit would be transmitted similarly, but at a different frequency, for example 129 kHz. Transmission of data from the IPG 10 to the external controller 100 occurs similarly via modulator/transmitter circuitry 47 in the IPG 10 and demodulator/receiver circuitry 106 in the external controller 100.
Wireless communications between the external controller 100 and IPG 10 can occur in different manners, and external controller can be differently configured, as explained in U.S. Patent Application Ser. No. 61/874,863, filed Sep. 6, 2013.
FIG. 4A shows an architecture for IPG 10, which is described in U.S. Patent Application Publication 2013/0331910. Shown with particular emphasis are the various power supplies in the IPG 10, which are shown with thicker lines. Primary battery 34 provides the main power supply voltage, Vbat, from which all other power supply voltages in the IPG 10 are derived. Because Vbat is relatively small (e.g., around 3 Volts, but dropping as it depletes over the IPG 10's lifetime), and because certain circuits in the IPG 10 require higher power supply voltages than Vbat may be able to provide, the IPG 10 includes boost circuitry. In particular, IPG 10 includes a first boost converter 52 and a second boost converter 70, both of which comprise DC-DC converters for converting Vbat to different power supply voltages, i.e., to Vup and V+, as explained further below.
The first boost converter 52 generates power supply Vup, which comprises the power supply for most of the circuitry in the IPG 10, including analog circuitry 62, digital circuitry 64 (including microcontroller 50), and memory 60. Vup may be regulated (per regulators 54, 56, and 58) to derive separate power supply voltages Va, Vd, and Vf dedicated to each of these circuits. In one example, Vup can equal approximately 3.2 Volts, with low-drop-out regulators 54, 56, and 58 producing power supplies Va, Vd, and Vf of approximately 2.8 Volts. As the particulars of analog circuitry 62, digital circuit 64, and memory 64 are described in the above-cited '510 Application, they are not elaborated upon here. Vup is monitored via a monitor and adjust block 53, which compares Vup to a reference voltage, Vref, to determine whether Vup is too low. If so, this block 53 via control signal boost1 instructs the first boost converter 53 to operate, as explained further below.
The second boost converter 70 is used to generate a different power supply voltage, V+, called the compliance voltage, for powering the current generation circuitry 74 that produces the therapeutic current pulses (Iout) at one or more of the electrodes 16. In FIG. 4A, such current generation circuitry comprises one or more Digital-to-Analog converters (DAC(s) 74) that provide current pulses of the prescribed magnitude, frequency, and duration in accordance with digital control signals (CNTR). Because the prescribed current pulses can differ from time to time for a given patient, or from patient to patient, V+ is not fixed, but is instead set at an optimal level that is not too low to provide the prescribed current pulses, nor too high as to waste battery 34 power. Specifically, V+ monitor and adjust circuit 76 monitors a voltage drop across the DAC(s) 74, which it uses to control the second boost converter 70 to generate a power supply voltage V+ of an appropriate magnitude in accordance with control signal boost2. Again, further details regarding compliance voltage generation can be found in the above-cited '510 Application.
Both the first boost converter 52 (producing Vup) and the second boost converter 70 (producing V+) can comprise the same basic circuitry as shown in FIG. 4B, which comprises a well-known inductor-based boost converter. When enabled via control signal boost1 or boost2, a pulse width modulator 80 modulates a pulse width (PW) of a clock signal (CLK), which is sent to the gate of a transistor 84. When the transistor 84 is on, current (I) passes through an inductor 82. When the transistor 84 is turned off, the current in the inductor 82 discharges through a diode 86 to a capacitor 88, whose top plate comprises Vup in the first boost converter 52, or the compliance voltage V+ in the second boost converter 70. Because the capacitor 88 was already charged to the battery voltage, Vbat, the additional charge from the inductor 82 boosts Vup or V+ to a value higher than Vbat, with diode 86 preventing this excess charge from dissipating backwards into the circuit. Capacitor 88, in addition to storing the charge, also filters and stabilizes Vup and V+. Thus, as the gate of transistor 84 oscillates between on and off, Vup or V+ continues to boost at a rate determined by the duty cycle of the gate pulse train. When control signal boost1 or boost 2 is disabled, oscillations at the gate of the transistor 84 are halted, which causes Vup or V+ to fall as charge is consumed by the circuitry to which these power supplies are connected. Of course, the particulars of the circuitry values used in the first and second boost converters 52 and 70 will differ in accordance with their different functions and the voltages they must produce. Control signals boost1 and boost2 may be digital or analog, and may comprise a digital or analog value indicating how “hard” the boost converter must work to produce the desired power supply voltage.
It is known that a boost converter has the potential to interfere with the telemetry circuitry operable in an IPG. See U.S. Patent Application Publication 2010/0211132, discussing this issue in the context of the second boost converter 70 that produces the compliance voltage, V+. This is because the boost converter, via the current I through its inductor 82, will produce a magnetic field 85 when it operates, which magnetic field 85 may couple to the communication coil 40 in the IPG. Even if the communication coil 40 has a high quality factor and good out-of-band noise rejection, the magnetic field 85 produced by inductor 82 may still have frequency components generally within the band of the communication coil (e.g., from 100 kHz to 150 kHz). Moreover, the frequency components present in magnetic field 85 can be difficult to control because they are dependent on the power supply voltage being produced by the boost converter at any given time. If interference by the inductor 82 is severe, telemetry may not be reliable. Interference by the inductor 82 during reception of data at the communication coil 40 is especially problematic, as the telemetry signal received by the communication coil 40 may be quite small in magnitude.